Tangent Zoom

Inspired by Marc Renault's use of the second graphics window for zooming. See his calculus applets at
http://webspace.ship.edu/msrenault/GeoGebraCalculus/GeoGebraCalculusApplets.html.
This sketch zooms in on the tangent at a point, or just at a point on a graph.
The left window controls the function graphed and the point on that curve. The right window controls the scale of the zoom.
EDIT: Jen Silverman convinced me that it was important to keep the zoom window 1:1, so I fixed that!